1. Field of the Invention
This invention relates to the field of precision resistor circuits using MOS transistors.
2. Background Art
Precision resistors are often used in integrated circuit applications that require high stability and accuracy. For these applications, normal resistors are not employed since their tolerance ratings are too poor. Normal diffused resistors in IC technology are usually formed with doped silicon or polysilicon. An example of the processing of a prior art diffused resistor is given in the following explanation. On an n-type silicon substrate, a p-well is diffused into the epitaxial layer and then a silicon dioxide layer is grown on the surface of the epitaxial layer. Contact areas are opened at the ends of the p-well of uniform resistivity. The p-well is L cm long, W cm wide and t cm thick. Thus, the resistance of the diffused layer is: ##EQU1## where .rho.=resistivity of the material .OMEGA..multidot.cm.
Due to processing variations, the length, width, thickness and resistivity of prior art resistors cannot be controlled with enough precision to fabricate resistors of high precision. Thus, prior art resistors have required the use of well-known techniques such as "laser-trimming" in order to adjust resistor dimensions and thus adjust the resistance level to the desired value. However, performance of these resistors can suffer if the trimming process is not tightly controlled. In addition, laser trimming is a post processing technique, and adds to the expense of manufacture of integrated circuits.
Thus, it is an object of the present invention to provide a precision resistor that does not require any post-processing.
It is another object of the present invention to provide a voltage-controlled variable precision resistor.
It is another object of the present invention to provide high value impedance in a small area.